1. Field of the Invention
The field of the invention generally relates to growing single crystal silicon by the Czochralski (CZ) technique. In particular, the field of the invention relates to a system and method for measuring crucible weight in order to control the introduction of feedstock resulting in stable and repeatable crystal growing conditions for producing uniform crystals efficiently.
2. Background of Related Art
Conventional Czochralski crystal growth processes generally deplete the melt in the crucible as the crystal is grown. The height of the top of the melt, known as the melt level, therefore drops during growth. The melt height with respect to the heaters, heat shields, and gas flow diverters is an important growth parameter, and is best kept constant for optimum stable crystal growth. One method of compensating for the variable melt height is to raise the crucible as the crystal is grown. This method is mechanically complex and also does not completely keep the growth conditions constant, since the crucible and its support structure still move with respect to the heaters, heat shields, and gas flow diverters. Another method for compensating for the melt depletion is to add more raw feedstock into the melt to replace the withdrawn material. It is best to have some measurement of the melt level and/or melt mass in order to control the feedstock introduction. One method of melt level measurement that has been used is optical reflection from the melt surface. Optical reflection from the melt is difficult because the melt is surrounded by radiation shields and insulation which may be closely positioned to the growing ingot. It is necessary to compromise the shields and insulation in order to provide an optical path for the measurement. Also, vibration of the melt causes fluctuations of the melt surface which can upset the melt level measurement. This is particularly troublesome since both the melt and crystal are generally rotating during crystal growth, adding to the potential for vibration.
In another method of Czochralski crystal growth the melt is allowed to substantially deplete, the crystal is withdrawn from the chamber, and then the crucible is recharged with solid feedstock. In this case the optical reflection method cannot be used to control the introduction of feedstock since there is no melt surface initially.
In all CZ growth processes (batch, batch recharged, or continuous), it is important to know and control the mass of silicon in the melt at the beginning of the process. The amount of silicon at the beginning of the process sets the boundary conditions for the heat flow and gas flow characteristics of the process which are to be matched from run-to-run to optimize grower performance. Slight variations in hardware setups (stack up tolerances for instance) can influence a measured melt height leading to error in determining silicon mass if the melt height reference is shifted accidentally leading to variations in heat flow characteristics.
For all of these reasons, it would be advantageous to have a reliable method of measuring the total amount of melt and feedstock in the crucible.